You can never know for sure, but I don't think there is such thing as a God or gods. People who say God exists because the Bible says so, make use of a fallacy, because it's supposed to 'promote' God and its content comes from God.
My guess is that the only reason why people believe in holy powers is that people don't understand everything and try to find an easy answer for it, just like what the ancient civilisations did. The Greek didn't understand how thunder comes into existance, so they 'made up' Zeus, the god of thunder for it.
Eventually people become a bit wiser and see that those gods don't exist and look for something else, which they find in one God, a fatherlike figure. Of course, now a lot of people don't believe in God... Or at least they don't say they believe in God, but rather in 'Intelligent Design', which is the idea that a certain being (in other words: a god) designed everything. Surely, the evolution theory doesn't tell me everything... The evolution of the eye for example... You can't just 'add' one part of an eye; an eye doesn't work [correctly] until everything is there. But I'm sure science will find an answer to that...
And the idea of a 'punishment' in the 'after life' if you did not believe in God or didn't live in a good way... Where did that come from? As far as I know, it isn't really named anywhere... The hell is something the church created in the Middle Ages, the time when they controlled everything.